Highly filled carbon nanofiber reinforced polysiloxanes

ABSTRACT

Provided herein are polysiloxane-based composite materials comprising a high weight percentage of elemental components (e.g., elemental boron, elemental copper, elemental bismuth, elemental lead) and low levels of carbon nanofibers. The elemental components may be present in the composite materials at levels greater than 25% by weight. Also provided herein are methods of forming the composite materials that have desirable flexibility and shielding properties. The invention provides a versatile composite material that is compliant and moldable, while still comprising high levels of the elemental shielding components. The materials are particularly useful for applications in which radiation shielding or neutron capture are desired.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the priority benefit of U.S. ProvisionalPatent Application Ser. No. 62/539,648, filed Aug. 1, 2017, entitledBORON-FILLED SILOXANE POLYMERS FOR RADIATION SHIELDING, incorporated byreference in its entirety herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.DE-NA0000622 and Contract No. DE-NA0002839, both awarded by the UnitedStates Department of Energy/National Nuclear Security Administration.The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is generally directed to polysiloxane-basedcomposite materials comprising a high weight percentage of elementalcomponents (e.g., elemental boron, elemental copper, elemental bismuth,elemental lead etc.) and carbon nanofibers. The materials areparticularly useful for applications in which radiation shielding orneutron capture are desired.

Description of the Prior Art

Boron is used in many industries for many unique applications, includingneutron shielding. However, to achieve many of the desired shieldingproperties, large loadings of boron are required. This oftennecessitates an extremely thick, low-density part or a very thin,high-density part, which is hard and brittle. Natural boron isunsuitable to be used as a structural component due to its inherentbrittleness. The ¹⁰B isotope can also be used in biochemically targetedradiotherapy approaches. In the Boron Neutron Capture Therapy (BNCT),the alpha particles and lithium ions formed from the ¹⁰B (n, alpha) ⁷Lireaction, give rise to closely spaced ionizing events that killcancerous cells while avoiding major damage to healthy ones. What isneeded is a versatile composite material that is compliant and moldable,while still comprising high levels of boron and/or other elementalshielding components.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a compositematerial. The composite material comprises polysiloxane polymer, carbonnanofibers, and greater than about 25% by weight of one or moreelemental components.

In another embodiment, the present invention is directed to a method ofproducing a composite material. The method comprises forming a mixturecomprising polysiloxane polymer, carbon nanofibers, and one or moreelemental components, and curing the mixture to produce the compositematerial. The composite material comprises greater than about 25% byweight of the one or more elemental components.

In yet another embodiment, the present invention is directed to acomposite material comprising from about 10% to about 75% by weight ofone or more polysiloxane polymers, from about 25% to about 90% by weightof one or more elemental components, and from about 0.1% to about 10% byweight of carbon nanofibers. The composite material is substantiallyfree of metal compounds and metalloid compounds other than the one ormore polysiloxane polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure (FIG. 1 is an SEM image showing that carbon nanofibers arewell-dispersed within the polymer matrix;

FIG. 2 is a graph showing FT-IR absorbance spectra for samples exposedto 122 MJ (top trace), 73 MJ (middle trace), and control sample (bottomtrace);

FIG. 3 is a graph showing ¹H NMR spectra of extracts obtained fromsamples exposed to 122 MJ, 99 MJ, 59 MJ, 47 MJ, and control sample, withresidual protons from the solvent showing up at 7.25 ppm, and from waterat 1.56 ppm;

FIG. 4 is a graph of TGA results showing AM pads undergoing mass lossduring a constant ramp in temperature, with the derivative weight (%/°C.) also illustrated, and the control indicated by dashed trace;

FIG. 5 is a graph showing DSC thermograms with cooling and heatingcycles obtained for samples exposed to 122 MJ (top trace), 99 MJ (secondtrace), 59 MJ (third trace), 47 MJ (fourth trace), and the controlsample (bottom trace);

FIGS. 6A and 6B are surface SEM images for pristine (FIG. 6A) and 122 MJ(FIG. 6B) samples;

FIGS. 7A and 7B are cross-sections SEM images for pristine (FIG. 7A) and122 MJ (FIG. 7B) samples; and

FIGS. 8A and 8B are graphs showing engineering stresses as a function ofthe engineering strains measured for the sample irradiated to 122 MJ(FIG. 8B) and the control sample (FIG. 8A).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is generally directed to composite materialscomprising (consisting of, or consisting essentially of) one or morepolysiloxane polymers and one or more elemental components. As usedherein, “elemental component” refers to a component material comprisingan element that is not combined with other elements to form a compound.For example, the elemental component may comprise a single element in amonatomic, diatomic, or allotropic form in a zero-oxidation state. Incertain embodiments, the composite material is substantially free ofmetal compounds and metalloid compounds other than the polysiloxanepolymer(s). As used herein, “substantially free” means that thecomposite material comprises less than about 1% by weight of thecompound. In certain embodiments, the one or more elemental componentsare present in the composite material at about 25% to about 90% byweight, preferably about 40% to about 75% by weight, and more preferablyabout 45% to about 60% by weight, with the total weight of the materialtaken as 100% by weight. In certain embodiments, the composite materialcomprises greater than about 25% by weight, greater than about 30% byweight, greater than about 35% by weight, greater than about 40% byweight, or greater than about 45% by weight of the one or more elementalcomponents. In certain embodiments, the composite material compriseseven higher loadings of the one or more elemental components, forexample, greater than about 60% by weight, greater than about 75% byweight, and greater than about 85% by weight of the one or moreelemental components. In certain embodiments, the one or more elementalcomponents are selected from the group consisting of elemental boron,elemental copper, elemental bismuth, elemental lead, and combinationsthereof. In certain embodiments, the one or more elemental componentscomprise a single elemental component. In certain embodiments, thecomposite materials further comprise carbon nanofibers. Therefore, incertain embodiments, the composite materials comprise (consist of, orconsist essentially of) one or more polysiloxane polymers, one or moreelemental components, and carbon nanofibers. The composite materials areparticularly useful for applications in which radiation shielding and/orneutron capture are desired.

Polysiloxane polymers (also known as polysiloxanes or silicones) arepolymers that comprise a chain of alternating silicon atoms and oxygenatoms, which can be combined with carbon and/or hydrogen. Polysiloxanesgenerally have the chemical formula [R₂SiO]_(n), where R is an organicgroup, such as an alkyl (methyl, ethyl) or phenyl group. In certainembodiments, the one or more polysiloxane polymer comprise aromaticgroups in the polymer backbone. In certain such embodiments, the one ormore polysiloxane polymers comprise polydimethylsiloxane (PDMS). Thiscopolymer advantageously exhibits good thermal and chemical stabilities,as well as chemical resistance to ionizing radiation due to the presenceof aromatic groups in the siloxane backbone. In certain embodiments, theone or more polysiloxane polymers are present in the material at about10% to about 75% by weight, preferably about 25% to about 60% by weight,and more preferably about 40% to about 50% by weight, with the totalweight of the material taken as 100% by weight.

In certain embodiments, the one or more elemental components compriseelemental boron. Elemental boron has two naturally occurring and stableisotopes, ¹¹B and ¹⁰B. The ¹⁰B isotope generally exhibits radiationshielding and neutron capture properties. In one or more embodiments,boron is present in the composite materials as enriched ¹⁰B boronfiller. Particularly, in certain embodiments, the boron contentcomprises greater than about 50% ¹⁰B, preferably greater than about 75%¹⁰B, and more preferably greater than about 90% ¹⁰B, and most preferablygreater than about 99% ¹⁰B, with the total boron content of thecomposite material taken as 100% by weight. In certain embodiments,elemental boron is present in the material at about 25% to about 75% byweight, preferably about 40% to about 60% by weight, and more preferablyabout 45% to about 55% by weight, with the total weight of the materialtaken as 100% by weight. In certain embodiments, the composite materialcomprises greater than about 25% by weight, greater than about 30% byweight, greater than about 35% by weight, greater than about 40% byweight, or greater than about 45% by weight of elemental boron. Incertain embodiments, the composite material is substantially free ofboron compounds (e.g., boric acid, borate, borax, boron oxide, boronnitride, etc.).

The one or more elemental components may comprise other componentsimparting similar or different properties as elemental boron into thecomposite materials. In certain embodiments, the one or more elementalcomponents comprise elemental copper, elemental bismuth, and/orelemental lead. These particular components impart shielding propertiesinto the composite material. In certain embodiments, elemental copperand/or elemental bismuth are present in the material at about 25% toabout 90% by weight, preferably about 40% to about 75% by weight, andmore preferably about 45% to about 60% by weight, with the total weightof the material taken as 100% by weight. In certain embodiments, thecomposite material is substantially free of copper compounds, bismuthcompounds, and/or lead compounds.

In certain embodiments, the composite materials comprise carbonnanofibers. Carbon nanofibers are cylindric nanostructures with graphenelayers arranged as stacked cones, cups, plates, or cylinders(nanotubes). In one or more embodiments, the carbon nanofibers aredispersed within the polysiloxane resin. The well-dispersed nanofiberslead to an “interface dominated” material with improved mechanicalproperties even at very high filler loadings. In certain embodiments,the carbon nanofibers are present in the material at about 0.1% to about10% by weight, preferably about 1% to about 5% by weight, and morepreferably about 2% to about 4% by weight, with the total weight of thematerial taken as 100% by weight.

Additional additive components may also be included as needed ordesired. However, in certain embodiments, the composite materialconsists essentially of (or consists of) a polysiloxane polymer and oneor more elemental components. In other embodiments, the compositematerial consists essentially of (or consists of) one or morepolysiloxane polymers, one or more elemental components, and carbonnanofibers.

Methods of forming the composite materials comprise mixing the one ormore polysiloxane polymers, one or more elemental components, and carbonnanofibers to form a composite material comprising the polysiloxanepolymer resin(s) with the elemental components and carbon nanofibersdispersed therein. The mixture may be prepared by adding a solid powderform of the one or more elemental components to the polysiloxanepolymer. In certain preferred embodiments, the mixture is formed byseparately adding the carbon nanofibers and the one or more elementalcomponents to the polysiloxane polymer. The polysiloxane polymer(s) maybe synthesized, for example, by mixing and/or crosslinking two polymersto form the polysiloxane polymer or copolymer resin. In certainembodiments, poly-dimethyl-vinyl terminated is mixed withpoly-methyl-hydrosiloxane to form the polydimethylsiloxane. In certainsuch embodiments, the poly-dimethyl-vinyl terminated andpoly-methyl-hydrosiloxane are mixed at a ratio of about 2:1 to about99:1, preferably about 5:1 to about 50:1, and more preferably about 8:1to about 20:1 poly-dimethyl-vinyl terminated topoly-methyl-hydrosiloxane. Aromatic substitution in the polymer backboneprovides remarkable protection against radiolysis with tolerance toionizing radiation, increasing almost exponentially with increasingphenyl concentration. Regardless the polysiloxane polymer or polymerprecursors provided, the carbon nanofibers and elemental component(s)are then mixed and dispersed therein. Any additive components can bemixed added at this point. The resin is then cured to form the compositematerial. In particularly preferred embodiments, the components aremixed as described above and cured by heating to a temperature of atleast about 105° C. using 0.10 g of platinum cyclovinylmethylsiloxanecomplex.

The composite materials described herein have a number of advantagesover prior art materials, which allows the materials to be used for anumber of applications. The polysiloxane polymer is reinforced withcarbon nanofiber, in contrast to the silica reinforced polymers of theprior art. This provides for structural reinforcement of the materials,while still allowing for inclusion of the needed levels of boron (orother component) for effective shielding and for production of aflexible and thin part that is ideal for many applications. The materialis flexible and compliant at high loading levels (e.g., high boronloading levels) and displays good mechanical properties at highconcentrations (e.g., high boron concentrations). Furthermore, inparticularly preferred embodiments, good resistance to ionizingradiation and effective capture of thermal neutrons is accomplished bythe use of a cross-linked copolymer with high concentrations of botharomatic groups in the polymer backbone and isotopically enriched B-10in the polymer matrix. Advantageously, the composite materials describedherein are easily moldable, even at high weight percent boron levels (orother elemental component levels). Unlike prior art materials, thecomposite material possesses a low volatile content, which reduces offgassing. Additionally, the ability of polysiloxanes (silicones) toprovide certain compliance properties when cured offer advantages overcurrent matrix materials. The material is customizable to obtain a widerange of densities and mechanical properties and exhibits high stabilityin the presence of neutron exposure.

Composite materials in accordance with embodiments of the presentinvention can be used for a variety of applications. Due to its largeneutron cross-section, the ¹⁰B isotope is an efficient shieldingmaterial for neutron and cosmic radiation. The composite materials areparticularly useful for applications where neutron shielding or neutroncapture is desired in a material that must be compliant and moldable.For example, the composite materials according to certain embodiments ofthe present invention can be used for shielding in nuclear power plants,for personal protection equipment, and for cosmic radiation protection.Specific applications for the composite materials comprising the isotopemay include shielding of neutron spectra in and around nuclear reactorsand in spent fuel pools. Space radiation shielding for long durationspace flight could be one of the many applications for the compositematerials comprising this isotope.

Additional advantages of the various embodiments of the invention willbe apparent to those skilled in the art upon review of the disclosureherein and the working examples below. It will be appreciated that thevarious embodiments described herein are not necessarily mutuallyexclusive unless otherwise indicated herein. For example, a featuredescribed or depicted in one embodiment may also be included in otherembodiments, but is not necessarily included. Thus, the presentinvention encompasses a variety of combinations and/or integrations ofthe specific embodiments described herein.

As used herein, the phrase “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itselfor any combination of two or more of the listed items can be employed.For example, if a composition is described as containing or excludingcomponents A, B, and/or C, the composition can contain or exclude Aalone; B alone; C alone; A and B in combination; A and C in combination;B and C in combination; or A, B, and C in combination.

The present description also uses numerical ranges to quantify certainparameters relating to various embodiments of the invention. It shouldbe understood that when numerical ranges are provided, such ranges areto be construed as providing literal support for claim limitations thatonly recite the lower value of the range as well as claim limitationsthat only recite the upper value of the range. For example, a disclosednumerical range of about 10 to about 100 provides literal support for aclaim reciting “greater than about 10” (with no upper bounds) and aclaim reciting “less than about 100” (with no lower bounds).

EXAMPLES

The following examples set forth the synthesis and testing of acomposite material prepared according to embodiments of the presentinvention. It is to be understood, however, that these examples areprovided by way of illustration and nothing therein should be taken as alimitation upon the overall scope of the invention.

Example I 1. INTRODUCTION

In this example, a highly boron-filled polysiloxane-based material wasformulated that also contains small amounts of carbon nanofibers, whichenhanced its mechanical strength. This material was subjected to veryhigh neutron fluences within very short time periods to evaluate itsradiological tolerance. These transient neutron environments wereprovided by the Annular Core Research Reactor (ACRR), which is based atSandia National Laboratories (Albuquerque, N. Mex.). Radiation-inducedchanges in chemistry, thermal properties, microstructure, and mechanicalresponse were assessed by a combination of diverse experimentaltechniques, which provided valuable insights on the tolerance of thisnew composite to harsh radiation environments.

2. MATERIALS, EXPERIMENTAL SET UP AND CHARACTERIZATION TECHNIQUES 2.1.Material Synthesis

The polydimethylsiloxane (PDMS)-based composite was synthesized using89.73 g poly-dimethyl-vinyl terminated (Gelest PDV 0535, M_(w)=47,500; %phenyl is 4-6%), mixed with 7.27 g poly-methyl-hydrosiloxane (Gelest HMS151, M_(w)=1,900 to 2,000; mole ratio of MeSiHO is 15-18). The organicfiller used was carbon nanofibers (3.0 g of Pyrograff PR-19-XT-PS),whereas the inorganic filler was 100 g of isotopically enriched ¹⁰B(99.80% purity and a B10/B11 ratio of 12:1 or 92.3% B10). SEM imagesshown in FIG. 1 illustrates that the fibers were well-dispersed into thepolymer matrix. The resin was cured at 105° C. using 0.10 g of platinumcyclovinylmethylsiloxane complex (Gelest SIP6832.2).

2.2. Experimental Set Up

The samples were exposed to various neutron/gamma fluxes at the AnnularCore Research Reactor (ACRR) at Sandia National Labs with the goal ofevaluating their radiological tolerance. The ACRR is a water-moderated,pool-type research reactor capable of steady-state, pulsed and tailoredtransient operations. It has a dry, 9-inch diameter cavity in the core'scenter and a 20-inch diameter external cavity. The polymer samples werelocated in the center cavity of the reactor and were exposed to shortpulses with energies varying from about 50 to 120 MJ. Samples were cutinto 1″ diameter and were secured in place by stapling on the outside ofcardboard tubes. The height of the cardboard tube was such as to exposethe samples to well-controlled conditions inside the center cavity.Thermocouples were mounted on the surface of a certain number ofsamples, and the temperature profile was monitored using a CADET dataacquisition system. Type K thermocouples used in this work werecertificated and calibrated from 0 to 300° C. Sulfur dosimetry and TLDswere fielded as passive dosimeters and used to determine neutronfluences and gamma doses.

Pre-test models were created using Abaqus CAE and MCNP6 to determine theneutron-gamma dose in the polymer discs and estimate the temperaturechange during reactor exposures. These models guided the test assemblylayout, with regard to height and radial placement of discs in thecentral cavity. Discs were fielded high in the cavity to allow for highenergy pulses while not exceeding the PDMS thermal degradation. MCNPdose profiles showed the outer edges and surfaces to absorb much largerdose than the internal regions of the discs, due to thermal neutronabsorption on the outer surfaces. Thermocouple transducers were fieldedat different regions of the discs to capture this gradient. The thermalresponse of the polymers was calculated through a radiation dose andheat capacity correlation to arrive at degrees per neutron. That valuewas then scaled to degrees per-reactor-MJ, based on the average numberof neutrons per fission and energy yield per fission, and then linearlyextrapolated to various (MJ) pulse intensities. The modeling predictionsdid not account for conductive heat transfer within the polymer and intothe assembly mounts.

2.3. Thermal Analyses

TGA experiments were carried out using a commercial TA InstrumentsQ5000. 10 to 20 mg samples were heated from 30° C. to 750° C. at 2°C./min under nitrogen atmosphere. DSC experiments were performed on a TAInstruments Q2000 DSC with a liquid nitrogen cooling (LNC) accessory.Approximately 5 to 10 mg sample were sealed in a Tzero aluminum hermeticpan. The cooling-heating cycle consisted of equilibrating the sample to35° C., cool down to −20° C. at 10° C./min, cool down to −150° C. at 5°C./min, isotherm at −150° C. for 15 minutes, heat up to 100° C. at 10°C./min. Both the temperature and heat of fusion were calibrated usingthe melting of high purity Indium. The pristine polymer wascharacterized by a melting temperature ca. −44.5° C. and a glasstransition temperature at about −120° C., which are typical forsiloxanes.

2.4. Chemical Characterization

IR absorption spectra were obtained with a Nicolet Avatar 360 benchtopFTIR operating in Attenuated Total Reflectance (ATR) mode. Data wascollected using a Smart DuraSampleIR ATR accessory equipped with adiamond crystal. The IR detector was deuterated tri-glycine sulfate(DTGS). All data was taken with a resolution of 8 cm⁻¹, and representthe average of 32 scans. Extractable material was obtained by immersing1.93 g sample in 3.1 g of deuterated chloroform overnight (99.6%deuterated from Acros Organics). Extracts were analyzed by liquid-state¹H NMR experiments using a Bruker Avance NMR spectrometer operating at500.13 MHz; proton signals were referenced to the residual protons fromthe solvent.

2.5. Solvent Swelling

Experiments were performed to estimate changes in cross-linkdensity/chain scission as a result of radiation exposure. Threespecimens for each condition were analyzed by this method. The samples(about 1.93 g) were first weighed for the initial dry weight and thenimmersed in toluene (Fisher Scientific, 99% purity). Swollen sampleswere periodically weighed during two weeks when saturation was reached.Equilibrium weights were recorded at the end of the experiment. Thesamples remained whole during and after solvent uptake. Dry weightsafter solvent swelling were about 2% lower than initial dry weights withthe solvent remaining visibly clear.

2.6. SEM Imaging

A FEI Scios Low Vacuum Dual Beam field emitter SEM was used tocharacterize flat surface and cross-sectioned views of pristine andirradiated samples. The samples were sprayed with dry nitrogen to removeloose particles from the surface. Ted Pella conductive silver paint wasused to keep the samples in place during examination. Images for theflat surface and cross-sectioned views were taken at 350×, 1000× and3500× with respect to the SEM monitor display for characterization ofmaterial, imaging voltages ranged between 2 kV and 5 kV. The Scios LowVacuum imaging detector (LVD) was used in a whole chamber pressure rangeof 10 to 50 pascals.

2.7. Mechanical Testing

Each of the irradiated pads was subjected to cyclic compression test inorder to identify how each level of dose affected the mechanicalproperties. Sample dimensions were approximately 22.94 mm diameter, and3.53 mm height, and an average density of 1.33 g/cc. The samples weretested at room temperature at a load rate of 0.03576 mm/s to ˜30%engineering strain. The compression tests were conducted with an MTSmodel 880 test frame with a second 100 lb load cell and an extensometerto control and measure displacement more accurately. The extensometerused adds a small additional load due to the spring constant of thedevice, but it can be measured and subtracted from the experimental dataif it is a high enough percentage of the overall load signal. Generally,the load increase is about 0.75 N/0.5 mm of displacement, and it islinear over its range of accuracy (0.5″ displacement range). This biasload was determined to be less than 3% of sample load for the pads. Theanalysis of the current results does not subtract away this known biaserror since this error is consistent for all tests. The benefit to usingthis extensometer is that it eliminates system compliance and hysteresisthat can be seen in the LDVT signal, thus reducing the uncertainty ofthe load/displacement for the entire system. The experiments were run byplacing the samples on optically flat tungsten carbide platens, centeredin the loading system, with no lubrication. The tests were run byfinding the contact of the platens and setting that as zerodisplacement, opening to a gap large enough to insert the samples, bringthem into near contact with the sample to increase the thermalconductivity into the material. Then run to a gap that is approximately60% engineering strain. This enables one to determine contact duringeach test, make sure the sample is fully unloaded between cycles andensure no pre-strain is put on the sample during set up.

3. RESULTS AND DISCUSSION

Newly formulated polymeric composites were characterized for radiationsusceptibility after being exposed in a pool-type reactor configured asa reference field commonly used for radiation damage studies. The totalenergy deposited in polymers exposed to reactor radiation is dependenton the influence of several interactions, with gamma rays and thescattering of fast neutrons being typically the main contributors. Inone of the first investigations on the effects of reactor radiation onhydrocarbon and fluorocarbon polymers, the authors demonstrated that thedeposited energy depends strongly on the polymer hydrogen density due tothe high ¹H neutron scattering cross section. A good example ispolyethylene, which absorbs a large amount of energy from neutronsbecause it has high proton density. Furthermore, the efficiency of eachtype of radiation is generally related to the linear energy transfer(LET, equivalent to the stopping power), which describes the rate atwhich energy is deposited in matter per unit distance. Essentially, LETeffects in polymers provide an indication of the reaction mechanismsinvolved, as well as a basis for the prediction of property changes in aradiation field. Over the years, the effects of different kinds ofionizing radiations and LETs on the radiolysis of polymers have been thesubject of numerous studies. In particular, one work provided acomprehensive review of the LET effect on the generation of molecularhydrogen, a common product formed from the radiolysis of hydrocarbonpolymers. That review involved comparing polymers irradiated by ⁶⁰Coγ-rays (LET ca. 0.2 eV/nm), with polymers irradiated using heavy ionswith energies ranging from 5 to 30 MeV, and LET values up to 800 eV/nmfor 10 MeV carbon ions. It was found that, for high-densitypolyethylene, isotactic polypropylene, poly(methyl methacrylate), andpolystyrene, hydrogen yields generally increased with increasing LET. Inaddition, hydrogen yields of polymers exposed to very high-energyion-beams tended to approach those generated by γ-rays. Comparableeffects were observed when poly(di-n-hexylsilane) (PDHS) was irradiatedusing a variety of high-energy ion beams, electron beams, and ⁶⁰Coγ-rays, with LET values ranging from 0.2 to 1620 eV/nm. Changes in thePDHS molecular weight indicated that the number of cross-links perabsorbed 100 eV or G(X) increased from 0.042 to 0.91 with increasing LETvalues. Furthermore, another study showed that the cross-linking rate ofpolystyrene specimens irradiated in γ-sources and in a nuclear reactorwas two- to threefold higher for the reactor exposure (G(X)=0.096 forfast neutrons, LET ca. 30 eV/nm and G(X)=0.034 for ⁶⁰Co γ-rays). Theauthors proposed that increased yields of cross-links at high LET couldbe accounted for by enhancement of second-order processes in overlappingspurs rather than competing first-order reactions.

Ionizing effects are thus expected to become more pronounced ifradiation of high LET is used. The polymeric material synthesized hereinwas subjected not only to gamma rays and fast neutrons generated by thenuclear reactor, but also to alpha radiation originated from the boronfiller itself. The boron filler used here is isotopically enriched with¹⁰B having large cross-section (3840 barns). The nuclear capture andfission reactions that occur when non-radioactive ¹⁰B is exposed tothermal neutrons yield alpha particles and recoiling ⁷Li nuclei.Typically, these charged secondary particles have very high LET values,and as a result deliver a large dose in a very small volume. There arefew published studies concerning the effect of this type of radiation onthe degradation of polymers. One of such studies compared radiationeffects of alpha particles (generated from ¹⁰B(n,α)⁷Li, LET ca. 280eV/nm) to γ-rays (LET ca. 0.2 eV/nm) on boric acid/polystyrene mixtures.The authors measured G(X) values of the mixture to be equal to 0.15 forthe alpha exposure, but only 0.018 for the γ-ray exposure.

The environment in which the polymer is irradiated can also influenceradiation chemistry mechanisms like chain scission, cross-linking, andoxidation. Irradiation induced-radicals often tend to react to formcross-links when polymers are irradiated under inert atmosphere.Increasing in cross-link density can induce significant changes inmechanical properties, such as increase in elastic modulus and hardnessaccompanied by reduction in elongation. On the other hand, the nature ofthe chemical reactions is altered when polymers are irradiated underreactive atmosphere like air, which was the case in this experiment. Inother words, oxygen tends to react with free radicals and initiateoxidative degradation processes that can cause material hardening viaoxidative cross-linking in parallel with scission reactions. Thepresence of oxygen in the polymer or in the atmosphere may lead toformation of stable peroxides or hydroperoxides, which are likely tocompete with cross-link reactions. Generally, the presence of oxygenoften increases the amount and/or rate of oxidative degradation of thepolymer. The balance between cross-linking and chain scission reactions,both under inert or oxidative conditions is generally material specificand hence of interest in polymer radiation chemistry studies.

The radiation produced in the ACRR involves gamma rays and neutrons withenergies ranging from thermal to ˜3 MeV energies. A detailed descriptionof this research reactor can be found in earlier publications. In theACRR experiments performed herein, the temperature of the sample wasmeasured in-situ, and it was found to increase in a non-linear fashionwith increasing radiation exposure. It is worth mentioning that thetemperature was somewhat dependent on how well the thermocouple wasembedded into the sample, and may thus have contributed to produce smallvariations in the reading temperature. In-situ temperatures were alsosimulated in pre-test models using Abaqus and MCNP calculations usingthe radiation dose and heat capacity correlation to providedegrees-per-reactor-MJ. This approach relies on the estimated number ofneutrons emitted from a measured reactor yield (MJ), rather than a moreprecise method that employs passive dosimetry foils to measure actualneutron population, which can be applied in post-test modeling.Estimated pre-test temperature values agree relatively well withexperimentally-measured temperatures, as shown in Table 1.

TABLE 1 Gamma doses, neutron fluences and pulse energies applied to thefilled polymers, sample temperatures (observed and estimated) andmolecular weights between cross-links obtained from solvent-swellingexperiments. Gamma Neutron Neutron Peak Simulated Dose Fluence FluencePulse Temp Temp 10⁴ M_(c) (kGy) 1 Mev (Si) >3 MeV (MJ) (° C.) (° C.)g/mol 4.00 4.01e+14  5.22e+13 47.07 88 82 5.1 5.33 4.90e+14  6.29e+1359.56 114 104 4.9 5.74 6.11e+14  7.87e+13 73.22 122 128 4.8 7.188.26e+14 1.022e+14 99.25 151 174 4.7 8.13 1.00e+15 1.301e+14 122.62 155214 4.3

Radiation induced cross-linking and chain scission reactions wereevaluated by the solvent-swelling experiment, which is one of thepreferred methods to gauge variations in the polymer network due to itseasiness to perform. The experiments performed herein showed a slightdecreasing in the swelling behavior of the irradiated samples (thepristine sample swelled 340% whereas the sample exposed to the 122.62 MJexposure swelled to 312%). The equilibrium weights obtained in toluenewere then applied to the Flory-Higgins model to estimate the averagemolecular weight between cross-links (M_(c)) for the polymer-fillercomposite:

$\begin{matrix}{M_{c} = \frac{{\rho_{e}\left( {v_{f}^{1/3} - {v_{f}/2}} \right)}v_{1}}{{\ln\left( {1 - v_{f}} \right)} + v_{f} + {\chi\; v_{f}^{2}}}} & {{eq}.\mspace{14mu}(1)}\end{matrix}$

where ρ_(e) is the density of the rubber, v₁=106.5 is the molar volumeof toluene, v_(f) is the volume fraction of polymer in the sample atequilibrium swelling, and χ=0.451 is the polymer-solvent interactionparameter. Molecular weights between cross-links are listed in Table 1and suggest a small increasing in cross-link density (which is inverselyproportional to M_(c)) with increasing radiation dose.

The irradiated polymers were analyzed using FTIR spectroscopy withresults shown in FIG. 2 . Spectra are representative of siliconeelastomers in general. Numerous investigators have observed that, for awide range of experimental conditions and dose rates, abstraction of Hand CH₃ radicals take place when PDMS is exposed to ionizing radiation.This leads to the evolution of hydrogen, methane and ethane. In additionto these gases, carbon dioxide and carbon monoxide are typicallydetected when the polymer is exposed to radiation in air. Abstraction ofH or CH₃ radicals will result in a decrease in intensity of the band atca. 2964 cm⁻¹, which corresponds to the C—H stretching in CH₃. Anotherexpected effect of radiolysis would be a decrease in intensity of theca. 1261 and 799 cm⁻¹ bands (CH₃ symmetrical bending and CH₃ rocking, inSi—CH₃). Finally, an increase in cross-link density has been associatedwith changes in the broad band at ca. 1092 cm⁻¹ and 1020 cm⁻¹, which arerelated to asymmetric and symmetrical stretching of Si—O—Si;respectively. The FTIR spectra of irradiated samples showed nosignificant chemical changes, indicating minor damage associated to theradiation field produced by the reactor.

NMR spectroscopy was used to probe changes in the material's chemistryas well. Extracted material was obtained from the copolymer with theobjective to isolate and detect low molecular weight fragments resultingfrom potential chain scission reactions and oxidative degradation.Extracts were analyzed by liquid-state ¹H NMR spectroscopy and werefound to be composed mainly of a large peak corresponding to methylgroups from PDMS chains, and small peaks associated with aromaticprotons from 7.2 to 7.6 ppm. See FIG. 3 . Quantitative analysis of theextracts showed no significant changes in their concentration withradiation exposure. No additional fragments were detected that could beassociated with chain scission reactions in agreement with cross-linkformation being the predominant effect of reactor radiation.

The thermal stability of the irradiated samples was probed by TGAexperiments with the evolving weight % shown in FIG. 4 . For allsamples, there were low and high temperature distinct regions of masslosses, which can be identified by studying peak locations in the DW %data. Small mass losses at 100° C. and 152° C. can be associated withphysi-sorbed water and evaporation of low-molecular weight compounds.More significant mass losses occur at around 270° C. and max out at 340°C. with 25% mass loss. The second high temperature mass loss maxes outat 480° C. Both high temperature losses are likely attributed tode-polymerization of the polymer backbone with evolution of cyclicoligomers, with the trimer being the most abundant product of thethermal degradation of PDMS in an inert atmosphere or under vacuum.Furthermore, the degradation rate and residual mass can vary over alarge range depending on factors such as impurities, water, fillers, andresidual catalyst. The degradation rate was very similar for irradiatedand control samples, suggesting no significant changes in the thermalstability with reactor radiation. In case a significant increasing incross-linking density was to take place, decreasing of the thermaldegradation rate should occur as a result of decreasing in chainmobility. Once thermal degradation ended, there was a small variation inthe residual solid mass, which we attributed to sample to samplevariations.

Exposure of polymers to ionizing radiation may change the degree ofcrystallinity and shift the glass transition temperature (T_(g)). DSC, acalorimetry method that measures heat flow as a function of temperature,was applied to investigate these effects with results shown in FIG. 5 .DSC thermograms of pristine or irradiated samples exhibited noexothermic peak either in the cooling cycle or an endotherm peak in theheating cycle. The absence of crystallization phenomenon is likely dueto the high concentration of phenyl groups that essentially preventchain conformational mobility limiting the reconfiguration of the chainsinto a crystalline state. Thus, the low temperature mechanical responseof the synthesized material, such as modulus, yield stress, and creep,will not be affected by radiation induced changes in crystallinitylevels. The T_(g) for irradiated and control samples is observed atabout −112° C., whereas for PDMS is −123° C. It has been shown that forrandom copolymers, Tg is related to the mole percent of diphenyl groups(x) by the following relationship: Tg=1.95x−123. Accordingly, thediphenyl mole fraction in the synthesized copolymer is 5.6%.

Changes in the microstructure were evaluated by SEM with representativeimages shown in FIGS. 6A, 6B, 7A, and 7B. Interestingly, the sampleexposed to the highest radiation dose showed a much rougher surface thancorresponding images of the pristine material. Much more of the fillerseems to be exposed on the surface of the highly irradiated samples.This is in agreement with the MCNP dose profiles, which showed the outeredges and surfaces to absorb much larger dose than the internal regionsof the samples, due to thermal neutron absorption on the outer surfaces.TGA results indicated that mass losses occur at temperatures as low as150° C., which is the in-situ temperature of samples irradiated to 122MJ. Thus, heating is likely to promote evaporation of low-molecularweight compounds more efficiently on the surface than in the bulk of thesample. Furthermore, when the siloxane-based foams were exposed tosimilar gamma doses as the ones supplied by the nuclear reactor, nosystematic changes in the microstructure of the foam could unambiguouslybe attributed to radiation exposure. Thus, it is possible that changeson the surface of the boron-composite are more likely due to the higherLET radiation produced by alpha particles than radiation produced bygamma exposure.

Mechanical tests were performed to probe changes in the material'sresponse triggered by exposure to reactor radiation. Stress-strain testswere run at room temperature with results of cyclic loading as shown inFIGS. 8A and 8B. The elastic regime of the bulk material is wellexceeded in the tests, as evidenced by the nonlinear shape of thestress-strain curve. It is interesting to note that a non-reversiblehysteresis is observed after the first loading cycle, which correspondsto the Mullins effect. This stress softening of the material is detectedfor all samples and is likely due to rearrangement of the boron fillerand neighboring polymer chains. In general, increasing in cross-linkdensity would be accompanied by stiffening of the material or increasingin the Young's modulus. The results show no significant changes in themechanical response of the irradiated composites when compared tocontrol ones.

4. CONCLUSIONS

The present work investigated the tolerance of a newly formulatedboron-filled PDMS-based composite to radiation generated by a nuclearreactor. The results demonstrated that the irradiated copolymerexperienced no significant chemical changes as evidenced by FT-IR andNMR. A slight decreasing in solvent swelling indicated that radiationexposure induced formation of cross-links, which we inferred to belargely promoted by the high LET alpha radiation. No significantmodifications to the thermal stability and mechanical properties of theirradiated boron-filled copolymers were observed. SEM experimentsdetected larger amounts of debris on the surface of the irradiatedmaterial than on the surface of control samples, which were likely dueto the thermal evaporation of low-molecular weight species produced byin-situ temperature rising. Compressive loading tests showed no changesin the stiffness response to controlled strains with increasing reactorradiation exposure. Overall, this new polymeric material exhibited goodnuclear survivability when exposed to very harsh environmentalconditions. The successful synthesis of this highly-filled cross-linkedpolymer was largely attributable to the incorporation of carbonnanofibers. The well-dispersed nanofibers lead to an “interfacedominated” material with improved mechanical properties even at veryhigh filler loadings. Furthermore, good resistance to ionizing radiationand effective capture of thermal neutrons was accomplished by using across-linked copolymer with high concentrations of both aromatic groupsin the polymer backbone and isotopically enriched B-10 in the polymermatrix.

Example II

After achieving high levels of elemental boron incorporation into thepolysiloxane matrix (EXAMPLE I), other elemental materials such ascopper, bismuth, and lead were incorporated into the matrix. Thedifferent elements shield different forms of radiation. These otherelemental materials were also able to be incorporated at high loadings,while maintaining the elasticity desired in the final part and flowproperties that allowed for simple molding. The different elementalmaterials have different densities, which enabled higher loadings to beachieved. For example, elemental bismuth was loaded to levels of about90% by weight. These loading levels should similarly provide shieldingcapabilities.

The invention claimed is:
 1. A composite material comprisingpolysiloxane polymer, carbon nanofibers, and greater than 75% by weightof elemental boron.
 2. The composite material of claim 1, wherein theelemental boron comprises greater than about 50% by weight ¹⁰B isotope,with the total boron content of the composite material taken as 100% byweight.
 3. The composite material of claim 1, wherein the materialcomprises greater than 75% to about 90% by weight of the elementalboron.
 4. The composite material of claim 1, wherein the polysiloxanepolymer comprises aromatic groups in the polymer backbone.
 5. Thecomposite material of claim 4, wherein the polysiloxane polymer ispolydimethylsiloxane (PDMS).
 6. The composite material of claim 1,wherein the material comprises greater than about 10% by weight of thepolysiloxane polymer.
 7. The composite material of claim 1, wherein thematerial comprises about 0.1% to about 10% by weight of the carbonnanofibers.
 8. The composite material of claim 1, wherein thepolysiloxane polymer, the carbon nanofibers, and the elemental boron areuniformly dispersed within the composite material.
 9. A method ofproducing the composite material of claim 1, the method comprising:forming a mixture comprising the polysiloxane polymer, the carbonnanofibers, and the elemental boron; and curing the mixture to producethe composite material.
 10. The method of claim 9, wherein the curingcomprises heating the mixture to a temperature of at least about 105° C.11. The method of claim 9, wherein the curing occurs in the presence ofa platinum-based catalyst.
 12. The method of claim 9, wherein formingthe mixture comprises adding a solid powder form of the elemental boronto the polysiloxane polymer.
 13. The method of claim 9, wherein formingthe mixture comprises separately adding the carbon nanofibers and theelemental boron to the polysiloxane polymer.
 14. The method of claim 9,wherein the polysiloxane polymer is synthesized by mixingpoly-dimethyl-vinyl terminated with poly-methyl-hydrosiloxane.
 15. Themethod of claim 14, wherein the poly-dimethyl-vinyl terminated and thepoly-methyl-hydrosiloxane are mixed at a weight ratio of about 2:1 toabout 99:1.
 16. A composite material comprising: from about 10% to about75% by weight of one or more polysiloxane polymers; greater than 75% byweight of elemental boron; and from about 0.1% to about 10% by weight ofcarbon nanofibers, wherein the composite material is substantially freeof metal compounds and substantially free of metalloid compounds otherthan the one or more polysiloxane polymers.
 17. The composite materialof claim 1, consisting essentially of the polysiloxane polymers, theelemental boron, and the carbon nanofibers.
 18. The composite materialof claim 16, consisting essentially of the polysiloxane polymers, theelemental boron, and the carbon nanofibers.
 19. The composite materialof claim 1, consisting of the polysiloxane polymers, the elementalboron, and the carbon nanofibers.
 20. The composite material of claim16, consisting of the polysiloxane polymers, the elemental boron, andthe carbon nanofibers.